Stereospecifiek en stereoselectief
Geplaatst op 09 juni 2010 - 14:45
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Geplaatst op 09 juni 2010 - 18:28
Stereospecificity is where the mechanism of the reaction only allows formation of one stereoisomer. So Sn2 is a great example of this because you can only have backside attack, which means that the mechanism only allows formation of the opposite stereochemical configuration.
A stereoselective reaction is one where the mechanism allows formation of both products, but one product is preferred over the other. So a good example of this is the Sharpless epoxidation (go to the wikipedia page for this, but don't try to understand it fully unless you want to). Anyway, the Sharpless epoxidation allows you to form an epoxide selectively onto one face of the double bond versus the other. But the agent that forms the epoxide, a hydroperoxide (a peracid), can theoretically react from both sides, but prefers to react from one face due to the conditions of the reaction. These conditions don't affect the mechanism of the reaction, but they direct the hydroperoxide towards one face or the other. The enantioselectivity of the Sharpless epoxidation is very high.
For stereospecific reactions, you cannot get a mixture because the mechanism precludes formation of a mixture. The stereospecificity is always 100%. For stereoselectivity, you can get a mixture, but 100% stereoselectivity is also possible, since the selectivity could be so good that none of the other isomer forms. However, in practice, you don't really see 100% stereoselective reactions, although many reactions are in the high 90s.
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Geplaatst op 09 juni 2010 - 20:12
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